Specifications Of The Spectrum - shelyak Lhires Lite Guide D'utilisateur

1.16.2 Specifications of the spectrum

With some care, you can watch for some interesting features in the
spectrum which can be explained by the optical path. For some of
them, you can even correct them.
Absorption lines are curved
This is due to geometrical distortion of the light path which goes
slightly off-axis the internal doublet in a Littrow spectroscope.
Phenomena can be more or less visible depending of the grating
position and how grooves are perpendicular or not to the optical path.
Lines are tilted
(not exactly perpendicular to the dispersion axis)
This means the grating grooves are not exactly parallel to the slit. It
could come from the internal mirror which is not at the nominal
position.
There are dark line acros the spectrum
This can have multiple causes:
Slit is dusty dispite the protective optical glass; you can dismantle
–
and clean it.
Slit is adjusted to the optical limit. This is a choice we made to
–
propose very thin slit which on one side provocates those dark line
due to imperfection into the slit edge, but on the other side make
the resolution of the instrument.
This effect can be amplified if the instrument is not toward the
–
Sun. Readjust the pointing direction if you see those lines
appearing.
Index doesn't stay in place
Spring pressure is adjustable by a screw/bolt on back of the
instrument. Look for next section.
Lhires Lite – User Manual DC0005C 55 / 60